The present invention relates to a thermal cutout and more particularly to a thermal cutout set to operate at a predetermined temperature for ensuring protection of electric heating appliances for household use, especially laundry irons.
This type of cutout is not intended to act as a device for regulating the temperature of the domestic appliance in which it is mounted but has the sole function of a safety device in the event that the regulating system has failed to operate and that the domestic appliance is therefore subjected to an abnormal temperature rise.
In order to carry out its function, the cutout must come into operation as soon as an unusual temperature is attained and before the appliance to be protected has sustained any damage. To this end, it is necessary to ensure good heat transfer between the heating element to be controlled and the cutout.
For this reason, the cutout is preferably in direct contact with the heating element to be controlled. However, in the case of small domestic appliances such as laundry irons, for example, the space available for putting the cutout in contact with the heating element is limited by reason of the different accessories which surround the heating element and which are necessary for the operation of the domestic appliance. Thus in the case of an electric steam-iron, the heating element is constituted by the sole-plate of the iron, a large part of which is made unavailable by the presence of the heating resistor, of the steam-generating zone and of the zone of contact with the regulating means.
In consequence, the cutout has to be miniaturized for maximum utilization of available space.
Taking cost requirements into account, it is also necessary to ensure that the cutout structure is compatible with very high speed mass-production assembly operations performed by means of automatic machines.
Miniaturized thermal cutouts having good heat-transfer characteristics are already known. In particular, U.S. Pat. No. 3,291,945 describes a thermal cutout comprising a heat-conducting casing, two terminals carried by an electrically insulating plug and an electrically conductive moving element maintained in contact with each terminal by a fusible pellet which is in turn in contact with the casing. However, in accordance with the teachings of this patent, the fusible pellet must be formed of electrically insulating material. If this condition is not satisfied, the conductors are electrically connected to the casing, thus constituting a permanent short-circuit or a connection to ground even when the pellet has melted by reason of a general temperature rise. Furthermore, it is known that electrically insulating fusible pellet are usually fabricated from organic material which sublimes slowly at the normal utilization temperatures of a domestic heating appliance. If a cutout of the type described in U.S. Pat. No. 3,291,945 is employed in a domestic heating appliance, the fusible pellet therefore sublimes progressively in the course of normal operation of the appliance and the cutout finally operates or "blows" when the domestic appliance has not been subjected to any abnormal increase in temperature. This operational fault condition makes it necessary to replace the cutout, which is incompatible with the market requirements of potential non-professional users of the appliance.
British patent Application No. 2,005,477 describes a rechargeable thermal cutout having rotary contacts and a relatively complex structure which is not compatible with very rapid mass-production assembly operations.